Method of producing an inertial sensor

ABSTRACT

The present invention discloses an inertial sensor comprising a planar mechanical resonator with embedded sensing and actuation for substantially in-plane vibration and having a central rigid support for the resonator. At least one excitation or torquer electrode is disposed within an interior of the resonator to excite in-plane vibration of the resonator and at least one sensing or pickoff electrode is disposed within the interior of the resonator for sensing the motion of the excited resonator. In one embodiment, the planar resonator includes a plurality of slots in an annular pattern; in another embodiment, the planar mechanical resonator comprises four masses; each embodiment having a simple degenerate pair of in-plane vibration modes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This divisional application claims the benefit under 35 U.S.C. §120 ofthe following U.S. patent application:

U.S. patent application Ser. No. 10/639,134, filed Aug. 12, 2003, nowU.S. Pat. No. 7,040,163, by Shcheglov et al. and entitled “ISOLATEDPLANAR GYROSCOPE WITH INTERNAL RADIAL SENSING AND ACTUATION”, whichclaims the benefit of U.S. Provisional Patent Application No.60/402,681, filed Aug. 12, 2002, and entitled “CYLINDER GYROSCOPE WITHINTEGRAL SENSING AND ACTUATION”, by Shcheglov et al. and U.S.Provisional Patent Application No. 60/428,451, filed Nov. 22, 2002, andentitled “DESIGN AND FABRICATION PROCESS FOR A NOVEL HIGH PERFORMANCEMESOGYRO”, by Shcheglov et al.

STATEMENT OF GOVERNMENT RIGHTS

The invention described herein was made in the performance of work undera NASA contract, and is subject to the provisions of Public Law 96-517(35 U.S.C. 202) in which the Contractor has elected to retain title.

This application is related to the following applications, which are allincorporated by reference herein:

U.S. patent application Ser. No. 10/370,953, filed Feb. 20, 2003, byChalloner et al., and entitled “ISOLATED RESONATOR GYROSCOPE WITH ADRIVE AND SENSE PLATE”;

U.S. patent application Ser. No. 10/410,744, filed Apr. 10, 2003, byChalloner et al., and entitled “ISOLATED RESONATOR GYROSCOPE WITHCOMPACT FLEXURES”;

U.S. patent application Ser. No. 10/865,344, filed Jun. 10, 2004, byHayworth et al., and entitled “MULTIPLE INTERNAL SEAL RINGMICRO-ELECTRO-MECHANICAL SYSTEM VACUUM PACKAGE”;

U.S. patent application Ser. No. 11/051,884, filed Feb. 4, 2005, byChalloner et al., and entitled “ISOLATED RESONATOR GYROSCOPE WITH ADRIVE AND SENSE PLATE”;

U.S. patent application Ser. No. 11/103,899, filed Apr. 12, 2005, byChalloner et al., and entitled “ISOLATED PLANAR MESOGYROSCOPE”;

U.S. patent application Ser. No. 11/192,759, filed Jul. 29, 2005, byShcheglov et al., and entitled “PARAMETRICALLY DISCIPLINED OPERATION OFA VIBRATORY GYROSCOPE”; and

U.S. patent application Ser. No. 11/199,004, filed Aug. 8, 2005, byShcheglov et al., and entitled “INTEGRAL RESONATOR GYROSCOPE”.

This application is related to the following patents, which are allincorporated by reference herein:

U.S. Pat. No. 6,823,734, by Hayworth et al., issued Nov. 30, 2004, andentitled “ELECTROSTATIC SPRING SOFTENING IN REDUNDANT DEGREE OF FREEDOMRESONATORS”;

U.S. Pat. No. 6,629,460, by Challoner, issued Oct. 7, 2003, and entitled“ISOLATED RESONATOR GYROSCOPE”;

U.S. Pat. No. 6,698,287, by Kubena et al., issued Mar. 2, 2004, andentitled “MICROGYRO TUNING USING FOCUSED ION BEAMS”;

U.S. Pat. No. 6,915,215, by M'Closkey et al., issued Jul. 5, 2005, andentitled “INTEGRATED LOW POWER DIGITAL GYRO CONTROL ELECTRONICS”;

U.S. Pat. No. 6,944,931, by Shcheglov et al., issued Sep. 20, 2005, andentitled “METHOD OF PRODUCING AN INTEGRAL RESONATOR SENSOR AND CASE”;and

U.S. Pat. No. 6,990,863, by Challoner et al., issued Jan. 31, 2006, andentitled “ISOLATED RESONATOR GYROSCOPE WITH ISOLATION TRIMMING USING ASECONDARY ELEMENT”.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to gyroscopes, and in particular toresonator microgyroscopes or inertial sensors and their manufacture.More particularly, this invention relates to isolated resonator inertialsensors and microgyroscopes.

2. Description of the Related Art

Mechanical gyroscopes are used to determine direction of a movingplatform based upon the sensed inertial reaction of an internally movingproof mass. A typical electromechanical gyroscope comprises a suspendedproof mass, gyroscope case, pickoffs, or sensors, torquers, or actuatorsand readout electronics. The inertial proof mass is internally suspendedfrom the gyroscope case that is rigidly mounted to the platform andcommunicates the inertial motion of the platform while otherwiseisolating the proof mass from external disturbances. The pickoffs tosense the internal motion of the proof mass, the torquers to maintain oradjust this motion and the readout electronics that must be in closeproximity to the proof mass are internally mounted to the case whichalso provides the electrical feedthrough connections to the platformelectronics and power supply. The case also provides a standardmechanical interface to attach and align the gyroscope with the vehicleplatform. In various forms gyroscopes are often employed as a criticalsensor for vehicles such as aircraft and spacecraft. They are generallyuseful for navigation or whenever it is necessary to autonomouslydetermine the orientation of a free object.

Older conventional mechanical gyroscopes were very heavy mechanisms bycurrent standards, employing relatively large spinning masses. A numberof recent technologies have brought new forms of gyroscopes, includingoptical gyroscopes such as laser gyroscopes and fiberoptic gyroscopes aswell as mechanical vibratory gyroscopes.

Spacecraft generally depend on inertial rate sensing equipment tosupplement attitude control. Currently this is often performed withexpensive conventional spinning mass gyros (e.g., a Kearfott inertialreference unit) or conventionally-machined vibratory gyroscopes (e.g. aLitton hemispherical resonator gyroscope inertial reference unit).However, both of these are very expensive, large and heavy.

In addition, although some prior symmetric vibratory gyroscopes havebeen produced, their vibratory momentum is transferred through the casedirectly to the vehicle platform. This transfer or coupling admitsexternal disturbances and energy loss indistinguishable from inertialrate input and hence leads to sensing errors and drift. One example ofsuch a vibratory gyroscope may be found in U.S. Pat. No. 5,894,090 toTang et al. which describes a symmetric cloverleaf vibratory gyroscopedesign and is hereby incorporated by reference herein. Other planartuning fork gyroscopes may achieve a degree of isolation of thevibration from the baseplate, however these gyroscopes lack thevibrational symmetry desirable for tuned operation.

In addition, shell mode gyroscopes, such as the hemispherical resonatorgyroscope and the vibrating thin ring gyroscope, are known to have somedesirable isolation and vibrational symmetry attributes. However, thesedesigns are not suitable for or have significant limitations with thinplanar silicon microfabrication. The hemispherical resonator employs theextensive cylindrical sides of the hemisphere for sensitiveelectrostatic sensors and effective actuators. However its high aspectratio and 3D curved geometry is unsuitable for inexpensive thin planarsilicon microfabrication. The thin ring gyroscope (e.g., U.S. Pat. No.6,282,958, which is incorporated by reference herein) while suitable forplanar silicon microfabrication, lacks electrostatic sensors andactuators that take advantage of the extensive planar area of thedevice. Moreover, the case for this gyroscope is not of the samematerial as the resonator proof mass so that the alignment of thepickoffs and torquers relative to the resonator proof mass change withtemperature, resulting in gyroscope drift.

Vibration isolation using a low-frequency seismic support of the case orof the resonator, internal to the case is also known (e.g., U.S. Pat.No. 6,009,751, which is incorporated by reference herein). However suchincreased isolation comes at the expense of proportionately heavierseismic mass and/or lower support frequency. Both effects areundesirable for compact tactical inertial measurement unit (IMU)applications because of proof mass misalignment under accelerationconditions.

Furthermore, the scale of previous silicon microgyroscopes (e.g., U.S.Pat. No. 5,894,090) can not been optimized for navigation or pointingperformance resulting in higher noise and drift than desired. Thisproblem stems from dependence on out of plane bending of thinepitaxially grown silicon flexures to define critical vibrationfrequencies that are limited to 0.1% thickness accuracy. Consequentlydevice sizes are limited to a few millimeters. Such designs exhibit highdrift due to vibrational asymmetry or unbalance and high rate noise dueto lower mass which increases thermal mechanical noise and lowercapacitance sensor area which increases rate errors due to sensorelectronics noise.

Scaling up of non-isolated silicon microgyros is also problematicbecause external energy losses will increase with no improvement inresonator Q and no reduction in case-sensitive drift. An isolatedcm-scale resonator with many orders of magnitude improvement in 3Dmanufacturing precision is required for very low noise pointing ornavigation performance.

Conventionally machined navigation grade resonators such as quartzhemispherical or shell gyros have the optimum noise and driftperformance at large scale, e.g. 30 mm and 3D manufacturing precision,however such gyros are expensive and slow to manufacture. Micromachinedsilicon vibratory gyroscopes have lower losses and better driftperformance at smaller scale but pickoff noise increases and mechanicalprecision decreases at smaller scale so there are limits to scaling downwith conventional silicon designs. Conventional laser trimming ofmechanical resonators can further improve manufacturing precision tosome degree. However this process is not suitable for microgyros withnarrow mechanical gaps and has limited resolution, necessitating largerelectrostatic bias adjustments in the final tuning process.

There is a need in the art for small gyroscopes with greatly improvedperformance for navigation and spacecraft payload pointing. There isalso a need for such gyros to be scalable to smaller, cheaper and moreeasily manufactured designs with lower mechanical losses in silicon andgreater 3D mechanical precision for lower gyro drift. There is stillfurther a need for such gyros to have desirable isolation andvibrational symmetry attributes while being compatible with planarsilicon manufacturing. There is a need for the gyroscope resonator andcase to be made of the same material, preferably silicon and allowingclose proximity of pickoffs, torquers and readout electronics. Finally,there is a need for such gyros to provide adequate areas for sensing anddrive elements in a compact form for lower gyro noise at small scale. Asdetailed below, the present invention satisfies all these and otherneeds.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide a planar resonatorsupported on a central rigid stem and with substantially increasedsensing capability by utilizing a short solid cylindrical resonator ordisc having a substantial useable internal resonator volume, allowingthe incorporation of significantly more sensing for the measurement ofdesirable resonator internal motion. This use of a planar element, suchas a disc, rather than a shell or ring results in substantial top andbottom surface areas and a large internal volume for mounting additionalsensors. A disc provides the same favorable modes for Coriolis sensingas a cylindrical or hemispherical shell.

A typical resonator of the invention for an inertial sensor includes aplurality of concentric rings and interleaved segments connecting theconcentric rings. The rings are typically circular, but can also be inother closed shapes. Importantly, this resonator structure can be usedwith or without internal electrodes for actuation and sensing.

A typical embodiment of the present invention comprises an inertialsensor including a planar mechanical resonator for substantiallyin-plane vibration and having a central mounting point, a rigid supportfor the resonator at the central mounting point. Excitation elements aredisposed within an interior of the resonator to excite in-planevibration of the resonator and sensing elements are disposed within theinterior of the resonator for sensing the internal motion of the excitedresonator. In one embodiment, the planar resonator includes a pluralityof slots in an annular pattern.

In one embodiment of the invention, multiple concentric alternatingsegments or slots can be cut or etched through the disc, reducing thein-plane stiffness of the resonator. In addition, the segmented orslotted disc also obtains an increased area available for capacitivesensing. The advantages of this structure include permitting largerdeflections with increased sensitivity. Alternatively, in-plane straingauges can be placed on the top and bottom surfaces of a solid disc toprovide a constant sensing area, regardless of the disc thickness.However, this approach will produce a resonator with an increasedin-plane stiffness when compared with the segmented or slotted approach.

Because the resonator is planar, it's manufacture is convenientlyfacilitated through known wafer manufacturing technologies. For example,the planar resonator can be produced by reactive ion etching (RIE) theresonator from silicon bonded in place on a supporting siliconbaseplate. Electrode support pillars and interconnect wiring can beetched and deposited on the baseplate before bonding. The etchingprocess can thus be used to simultaneously produce the torquerexcitation and pickoff sensing electrodes along with the resonator and aportion of the gyroscope case, including a wall surrounding theresonator and an endplate. A third silicon wafer comprising the readoutelectronics and electrode interconnections can also be bonded to theresonator to complete the gyroscope case.

In addition, in further embodiments of the invention, the planarresonator can comprise four masses, each having a simple degenerate pairof in-plane vibration modes and all being centrally supported. Theplanar mechanical resonator can be designed with two degenerate in-planesystem modes producing symmetric motion of the four masses for Coriolissensing.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIG. 1A depicts a top view of an exemplary planar resonator gyroscope ofthe present invention;

FIG. 1B depicts a side view of an exemplary planar resonator gyroscopeof the present invention;

FIG. 1C illustrates a pattern for an exemplary planar resonator of thepresent invention;

FIG. 1D illustrates electrode operation for a first mode of an exemplaryresonator.

FIGS. 2A-2B represent a finite element model of an exemplary segmentedplanar resonator gyroscope of the present invention;

FIGS. 3A-3J illustrate strain energy within a finite element model of anexemplary solid planar resonator gyroscope in various modes;

FIGS. 4A and 4B depict another finite element model of an exemplarysegmented planar resonator gyroscope of the present invention;

FIGS. 5A-5C illustrate masks that can be used in producing an isolatedresonator of the invention;

FIGS. 6A-6C depicts various stages of an exemplary manufacturing processfor the invention;

FIG. 6D shows an exemplary resonator with a quarter cutaway to revealthe embedded electrodes;

FIG. 6E shows an exemplary gyro without end cap readout electronicsassembled

FIG. 7 is a flowchart of an exemplary method of producing a resonatoraccording to the present invention;

FIG. 8 illustrates an alternate isolated planar resonator gyroscopeembodiment comprising four masses vibrating in plane;

FIGS. 9A and 9B illustrate the Coriolis sensing modes of the four massesvibrating in plane;

FIG. 10A-10C illustrate the integration of a end cap wafer with thebaseplate including an integral vacuum cavity wall;

FIG. 10D illustrates a further embodiment with electrical feedouts tothe excitation sensing and bias electrodes provided on the end capwafer;

FIG. 10E illustrates yet another embodiment where the end cap waferincludes the control electronics for the gyro; and

FIG. 10F illustrates the integrated end cap wafer including the controlelectronics.

DETAILED DESCRIPTION

In the following description of the preferred embodiment, reference ismade to the accompanying drawings which form a part hereof, and in whichis shown by way of illustration a specific embodiment in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

1.0 Overview

Embodiments of the present invention generally describe an isolatedplanar vibratory gyroscope. Generally, embodiments of the inventionemploy embedded sensing and actuation providing a planar micromachinedsilicon gyroscope having desirable axisymmetric resonator with singlecentral nodal support, integral (and distributed) proof mass andflexural suspension and extensive capacitive electrodes with large totalarea. Advantageously, the entire resonator, embedded electrodes andintegral case wall of the present invention can be fabricated from asingle wafer of silicon.

Silicon ring resonators (e.g., U.S. Pat. No. 6,282,958) do not havelarge area internal capacitive sensors and actuators and requireflexible support beams. Other quartz hemispherical resonator gyroscopesare three dimensional so they cannot be micromachined and do not haveembedded electrodes. Although post mass type resonator gyroscopes havehigh angular gain, large area sensing elements and hence superior noiseperformance to other designs, they do not have the optimized resonatorisolation properties of a single central nodal support and often employa discretely assembled post proof mass. Further, integrally made, fullydifferential embedded electrodes as with the present invention,desirable for better thermal and vibration performance, are not possiblewith a discrete post proof mass resonator gyroscope or out of planegyroscope.

The principal problems with ring gyroscopes are the inherently smallsensor area around a thin ring and the flexibility or interaction of thesupport beams. A three dimensional hemispherical gyroscope has tallersides for large area capacitive sensing, but still requires assembly ofa discrete circumferential electrode cylinder or cup for sensing andexcitation. A tall cylinder with central support and circumferentialelectrodes also faces this problem. A short solid cylinder or disc witha central support and piezoelectric and/or electromagnetic wire sensorsand actuators, mounted to the top or bottom surface of the disc solvesthe problem of non-embedded sensors with small area. However, apreferred embodiment of this invention is a multiply slotted discresonator with capacitive sensing and actuation illustrated in exemplaryembodiment described hereafter.

2.0 Exemplary Planar Resonator Gyroscope Embodiment

FIG. 1A depicts a schematic top view of an isolated resonator for thegyroscope or inertial sensor embodiment of the present invention. Thegyroscope comprises a unique planar resonator 100 which is supported bya rigid central support 106 and designed for in-plane vibration. In theexemplary embodiment, the resonator 100 comprises a disc that includes anumber of slots, e.g. 116A-116D (generally referenced as 116) formedfrom concentric circumferential segments 104A-104E. The circumferentialsegments 104A-104E are supported by radial segments 102A-102E. Theoverall diameter of the resonator can be varied depending upon theperformance requirements. For example, a 16 mm diameter resonator canprovide relatively high machining precision and low noise. Furtherrefinement of the resonator can yield a resonator diameter of only 4 mmat significantly reduced cost.

FIG. 1B depicts a schematic side view of an exemplary isolated resonator100 of the present invention assembled into a baseplate 112. The centralsupport 106 supports the resonator 100 on the baseplate 112. At leastsome of the slots 116 in the resonator 100 provide access for theembedded electrodes 108A-108D which are also supported on pillars 114 onthe baseplate 112. The electrodes 108A-108D form capacitive gaps110A-110H (outward gaps 110A, 110C, 110F and 110H and inward gaps 110B,110D, 110E and 110G) with at least some of the circumferential segments104A-104E of the resonator 100. These electrodes 108A-108D provide forradial excitation of the resonator 100 as well as sensing motion of theresonator 100. To facilitate this each of the electrodes 108A-108D isdivided into multiple separate elements to improve control and sensingof the resonator. For example, the annular electrode 108B as shown canbe divided into two or more elements, at least one acting across theoutward gap 110C and at least one acting across the inward gap 110D.Vibration is induced in the resonator by separately exciting theelements to produce a biased reaction on the resonator 100 at theelectrode 108B location.

In general, the excitation electrodes 108B, 108C are disposed closer tothe central support 106 (i.e., within inner slots of the resonator 100)than the electrodes 108A, 108D (i.e. within outer slots of the resonator100) to improve sensing. However, the arrangement and distribution ofthe excitation and sensing electrodes 108A-108D can be varied asdesired. In further embodiments, additional electrodes can also be usedto bias the resonator 100 providing electrostatic tuning. Such biasingelectrodes can also include multiple separate elements as the excitationand sensing electrodes.

FIG. 1C illustrates a pattern 120 for an exemplary planar resonator 100of the present invention. This pattern 120 employs numerous concentricinterleaved circumferential slots 122. Some of the slots, e.g. 122A-122Eare wider to accommodate multiple element electrodes. For example, twoof the outer rings of wider slots 122A, 122B are for the sensingelectrodes and three of the inner rings of wider slots are for thedriving electrodes. The remaining slots 122 can serve to structurallytune the resonator 100 (e.g., lower the frequency) and/or they may beoccupied by bias electrodes which are used to actively bias theresonator in operation. The resonator and modal axes 124 are indicated;operation of the resonator identifies them as the pattern 120 issymmetric.

Although the exemplary resonator 100 is shown as a disc, other planargeometries using internal sensing and actuation are also possibleapplying principles of the present invention. In addition, the singlecentral support 106 is desirable, providing complete isolation of theresonator, however, other mounting configurations using one or moreadditional or alternate mounting supports are also possible. Forexample, although central mounting is desirable, in other embodiments ofthe invention the resonator can be alternately mountedcircumferentially.

As employed in the resonator 100 described above, a centrally supportedsolid cylinder or disc has two degenerate in-plane modes suitable forCoriolis sensing, however the frequencies are very high (greater than100 KHz) and the radial capacitance sensing area diminishes withcylinder height or disc thickness. However, the multi-slotted discresonator 100, shown in FIGS. 1A and 1B overcomes these problems. Byetching multiple annular slots through the cylinder or disc twoimmediate benefits result: two degenerate modes suitable for Coriolissensing with low frequency (less than 50 KHz) and large sense, bias anddrive capacitance. The low frequency derives from the increased in-planecompliance provided by the slots. The large sense, bias and drivecapacitance is a consequence of the large number of slots that can bemachined into the resonator.

FIG. 1D illustrates electrode operation for a first mode of theresonator of FIG. 1C. The electrodes 124 that operate with a resonator100 of the pattern 120 are shown in the left image. Four groups ofelectrodes 124 are used, each at a 90° interval around the circumferenceof the pattern. The positive excitation elements 126 and negativeexcitation elements 128, paired elements of the excitation electrodes,are driven to excite the resonator 100. These paired elements 126, 128share a slot with the positive elements 126 in the outward position andthe negative elements 128 in the inward position. Note also that asshown some of the pairs share a common slot with other distinctelectrode pairs, illustrating that multiple separately operableelectrodes can share a common resonator slot. The sensing electrodes aredisposed at a larger radial position and include positive sensingelements 130 and negative sensing elements 132 which together provideoutput regarding motion of the resonator 100.

A uniform radial spacing between slots 116, 122 can be employed, butother spacing may also be used, provided two degenerate in-plane modessuitable for Coriolis sensing are maintained. In addition, in furtherembodiments, some or all of the segments 104A-104E can be furtherslotted such that a single beam segment is further divided into acomposite segment including multiple parallel segments. Selective use ofsuch composite segments can be used to adjust the frequency of theresonator as well as eliminate harmful thermoelastic effects on driftperformance as the segments are stressed in operation of the resonator.Generally, adding slots to form composite circumferential segmentslowers the resonator frequency. The effect of machining errors is alsomitigated with multiple slots. Although such composite segments arepreferably applied to the circumferential segments 104A-104E, thetechnique can also be applied to the radial segments 102A-102E or otherdesigns with other segments in other resonator patterns.

Employing the in-plane design described, embodiments of the presentinvention obtain many advantages over other out-of-plane gyros. Forexample, the central support bond carries no vibratory loads,eliminating any friction possibility or anchor loss variability. Inaddition, simultaneous photolithographic machining of the resonator andelectrodes is achieved via the slots. Furthermore, diametral electrodecapacitances can be summed to eliminate vibration rectification andaxial vibration does not change capacitance to a first order. Modalsymmetry is also largely determined by photolithographic symmetry notwafer thickness as with other designs. Isolation and optimization ofsense capacitance (e.g., from the outer slots) and drive capacitance(e.g., from the inner slots) is achieved. Embodiments of the inventionalso achieve a geometric scalable design to smaller or larger diametersand thinner or thicker wafers. In addition, embodiments of the inventioncan be entirely defined by slots of the same width for machininguniformity and symmetry. Implementation of the present invention canalso accommodate silicon anisotropy producing frequency splits. Forexample, a <111> silicon wafer and/or a varied slot width can be used.

As mentioned above, high thermoelastic damping due to vibrationfrequency proximity to thermal relaxation resonance can result in shortresonance decay times and high gyro drift. However, the slot radialspacing can be adjusted to define an optimum beam width and a number ofslots can be additionally etched in between the slots defining theelectrode gaps to further reduce the vibrating beam width.

In further embodiments of the invention, the multiple ring structurewith staggered or interleaved radial spokes or segments, such asillustrated in FIG. 1A can be used without internal sensing/actuation.This resonator architecture can provide the advantages of averaging ofmachining errors, higher natural frequency with thinner silicon rings,higher Q (lower thermoelastic damping) and higher angular gain whencompared with resonators employing a single ring and “wagon wheel”spokes from a central hub, such as the ring resonator of U.S. Pat. No.6,282,958. The utility of this resonator structure is to providemultiple thin silicon rings with useful sturdy support to a central hub.Such a resonator can be employed whether or not internal actuation andsensing is also used. Furthermore, although it is desirable to employ acentral mounting point, this resonator architecture can alternately bemounted from its periphery (e.g. its circumference) or using one or moreother mounting points. Staggering or interleaving the radial segmentsindicates not all the radial segments form straight lines from thecenter of the resonator to the periphery (although some may). It shouldalso be noted that the term “ring” as used herein does not require acircular shape. For example, the circumferential segments forming theconcentric “rings” of the resonator of FIG. 1A actually form a polygon.Circular rings are desirable, but any closed shape can also be used.

3.0 Planar Isolated Resonator Model

FIGS. 2A and 2B depict a finite element model of an exemplary slotteddisc planar resonator gyroscope of the present invention. FIGS. 3A-3Jillustrate strain energy within a finite element model of an exemplarycylinder resonator gyroscope in various modes. These various solidcylinder resonator modes illustrate radial strain patterns in a solidcylindrical resonator vibration, aiding in the development of thesegmented embodiment of the present invention, such as shown in FIGS.2A-2B.

FIGS. 4A and 4B depict another finite element model of an exemplarysegmented planar resonator gyroscope of the present invention. Themulti-slotted disc Coriolis coupled modes are depicted. Strain contoursare shown with a fixed central support in white. In this case, a largernumber of segments are employed than in the model of FIGS. 2A and 2B.

4.0 Producing an Isolated Planar Resonator Gyroscope

FIGS. 5A-5C illustrate masks that can be used in producing an isolatedresonator of the invention. FIG. 5A illustrates a top view of themulti-slotted disc resonator fabrication pattern 500. The resonatorpattern includes a large central area 502 which is bonded to the centralsupport on the baseplate. The embedded electrodes, e.g. concentricannular electrodes 504A-504F, are defined by the through etching processthat simultaneously defines the structure 506 (radial andcircumferential segments) of the resonator. FIG. 5B illustrates a topview of the multi-slotted disc baseplate pattern 508 showing the bondingpads, e.g., electrode bonding pads 510A-510F and the central supportbonding pad 512. FIG. 5C illustrates a top view of the multi-slotteddisc resonator bonded to the baseplate. To illustrate the alignment, theelectrode bonding pads 510A-510F and central support bonding pad 512 areshown through the electrodes and resonator structure 506, respectively.Known silicon manufacturing processes can be employed.

For a mesoscale (16 mm) gyroscope, a 500 micron wafer, e.g. silicon, canbe through-etched with circumferential slot segments to define a planarcylindrical resonator with embedded electrostatic sensors and actuators.Integral capacitive electrodes can be formed within these slots from theoriginal resonator silicon during the through etch process. This can beaccomplished by first bonding the unmachined resonator disc to a basesilicon wafer that is specially prepared with circumferential bondingpillar segments to support the stationary electrodes and centralresonator. The pillar heights may be defined by wet chemical etching andgold compression or gold-silicon eutectic bonding can be used to bondthe resonator to the support pillars before the resonator and itselectrodes are photolithographically machined using deep reactive ionetching (DRIE). In addition, for a microscale (4 mm) resonator a 100micron thick silicon wafer or silicon on insulator (SOI) or epitaxialsilicon layer is required for the resonator wafer. Alternatively, athicker wafer can be bonded to the baseplate and ground down andpolished to the desired thickness. The dense wiring can be photographedonto the baseplate before resonator bonding and wirebonded outside thedevice to a wiring interconnect grid on a ceramic substrate inconventional vacuum packaging or interconnected to a readout electronicswafer via vertical pins etched into the resonator for a fully integratedsilicon gyroscope that does not require a package.

FIGS. 6A-6C depicts various stages of an exemplary manufacturing processfor the invention. FIG. 6A shows a sequential development of thebaseplate for the resonator gyroscope. The process begins with a wafer600, e.g. a 500 micron silicon wafer as shown in the first image. Thewafer 600 is first etched to produce electrode pillars 602 as well as acentral resonator support pillar 604, such as for a single centralsupport as shown in the second image. The etching process can includegrowing a wet oxide, e.g. silicon oxide, approximately 1500 Angstromsthick, followed by pillar mask lithography according to standardtechniques (e.g. using an AZ 5214 resist) known in the art. This isfollowed by a buffered oxide etch (BOE) of the mask, such as with aSurface Technology Systems (STS) to a depth of approximately 20 microns(corresponding to approximately 5 to 6 minutes). The resist is thenremoved, using an O₂ ashing process for example. Next, the wafer 600 isBOE dipped for approximately 10 to 20 seconds to remove the nativeoxide. Following this is a potassium hydroxide (KOH) dip ofapproximately 1 minute at 90° C. The oxide is then removed with HF. Thethird image of FIG. 6A shows an oxide layer 606 applied over the etchedwafer 600. This layer 606 can be applied by growing a wet oxide forapproximately 3 days at approximately 1050° C. The oxide layer 606 canthen be etched, e.g. using a AZ 5740 resist, BOE oxide etch forapproximately 2 hours to get through approximately 5 microns of oxide.After removing the resist, a metalization layer is then applied to formbonding pads 608, 610 on each of the pillars 602, 604, respectively, asshown in the fourth image of FIG. 6A. Application of the metalizationlayer can be accomplished by mask lithography, depositing the mask (e.g.AZ 5214 at 2000 RPM for 20 seconds). The metal, e.g. 100 Angstroms Ti,200 Angstroms Pt and 3500 Angstroms Au, is then deposited and the maskis lifted off to yield metal bonding pads 608, 610 only on the surfacesof the pillars 602, 604. Finally, the processed wafer 600 is washedthoroughly with an O₂ ashing process prior to bonding with the resonatorwafer.

Application of the metalization layer for the bonding pads can alsoinclude patterning of the electrical wiring from the electrodesphotographed onto the baseplate, wirebonded outside the device to awiring interconnect grid on a ceramic. Alternately, in further novelembodiments discussed hereafter, the electrical wiring can bealternately developed into an integral vacuum housing producedsimultaneously with the resonator.

FIG. 6B shows a sequential development of the resonator wafer for thegyroscope. The first image shows the uniform thickness wafer 612, e.g.of silicon, used to form the resonator. The wafer 612 can first have theback side processed to produce alignment marks with mask lithographyapplying a resist. The alignment marks can be produce through a reactiveion etch (RIE) process using CF₄ and O₂ until a relief is clearlyvisible (approximately 5 to 10 minutes). Alternately, an STS process forapproximately 1 minute can also be used. After removing the resist,metalization lithography used to apply a mask to the front side of thewafer 612 to produce bonding pads 614, 616. The metal, e.g. 30 AngstromsCr and 3500 Angstroms Au or 100 Angstroms Ti, 200 Angstroms Pt and 3500Angstroms Au, is applied and the mask is lifted off to reveal thebonding pads 614, 616.

FIG. 6C shows integration of the resonator and baseplate wafers andformation of the functional resonator for the gyroscope. Thepreprocessed baseplate wafer 600 and resonator wafer 612 are bondedtogether as shown in the first image of FIG. 6C after aligning the twowafers 600, 612 to approximately 1 micron. Bonding fuses the metalbonding pads (the electrode pads 608, 610 as well as the central supportbonding pads 614 and 616) to form single bonded metal joints 618 and canbe performed at approximately 400° C. and 5000 N. Next, the completeresonator 630 and electrodes 632A, 632B (generally referenced as 632)are simultaneously formed directly from the bonded structure by throughetching. The through etching process can be performed using deepreactive ion etch (RIE) such as a suitable STS process with aphotolithographically defined mask, e.g. an AZ 5740 mask, approximately6 to 8 microns thick. The mask can be made as thin as possible forthrough etching. The resonator wafer 612 is then through etched over themask pattern to simultaneously produce the resonator 630 as well as theseparate electrodes 632A, 632B from the original wafer 612. See alsoFIGS. 5A-5C. Note that single electrodes 632 can be formed by throughetching, forming passages 622, 624, to isolate a section of theresonator wafer 612 attached to a bonded joint 618. In addition, asdiscussed above with respect to FIG. 1B, electrodes can also be dividedinto multiple separate elements. For example, through etching anadditional passage 626 separates the electrode 632 into two isolatedelectrode elements 632A, 632B. In this case the passage 626 mustpenetrate the metal bonded joint 618 to isolate the separate electrodeelements 632A, 632B. At the conclusion of the through etching process,the resonator 630 structure is only supported at the central resonatorsupport pillar 604.

FIG. 6D shows an exemplary resonator 630 with a quarter cutaway toreveal the embedded electrodes 632. A dust ring 634 is also shown thatcan be etched along with the resonator 630. Intermittent gaps 636 in thedust ring can be made to accommodate metal traces to the electrodes 632to operate the gyro.

Elements of the exemplary planar silicon resonator gyro embodimentspresented herein can be assembled with conventional vacuum packaging anddiscrete electronics in a manner similar to previous gyros. An internalceramic substrate wiring bonded to the silicon gyro baseplate can bechanged to match the new and old designs to existing packages.

FIG. 6E shows an exemplary gyro in a typical packaging assembly. Metaltraces 638 from the electrodes 632 of the resonator 630. The dust ring634 with intermittent gaps 636 allows passage of the metal traces 638 onthe baseplate wafer 600 to the electrodes 632. The metal traces 638 leadto vertical connect pins 640 which pass through the baseplate wafer 600(providing a vacuum seal). In the exemplary architecture shown, thevertical connect pins 640 are disposed in the corners of the squarebaseplate wafer 600. A vacuum cavity wall 642 surrounds the entireassembly. The vacuum cavity wall 642 can be applied as a part of aconventional housing covering the resonator 630 and bonded to thebaseplate wafer 600. Alternately, in further embodiments discussedhereafter, an vacuum cavity wall 642 can be produced simultaneously withthe resonator.

FIG. 7 is a flowchart of an exemplary method of producing a resonatoraccording to the present invention. The method 700 includes providing aplanar mechanical resonator for substantially in-plane vibration andhaving a central mounting point at step 702. The resonator is supportedat the central mounting point at step 704. At step 706, at least oneexcitation electrode is provided within an interior of the resonator toexcite in-plane vibration of the resonator. Finally at step 708, atleast one sensing electrode is provided within the interior of theresonator for sensing the motion of the excited resonator. As discussedabove, in further embodiments the planar mechanical resonator can beprovided and supported substantially simultaneously with providing theexcitation and sensing electrodes by through etching.

Final mechanical trimming with a laser or a focused ion beam (FIB) ofall embodiments described herein can be optionally used to achieve fullperformance over thermal and vibration environments. Such FIB tuningapplied to gyro resonators is described in U.S. application Ser. No.10/285,886 filed Nov. 1, 2002 by Kubena et al., which is incorporated byreference herein.

5.0 Alternate Isolated Planar Resonator Gyroscope

An exemplary centrally supported planar resonator having concentriccircumferential slots with internal electrodes to produce substantiallyin-plane vibration is described above. However, it is important to notethat other planar resonator patterns are also possible using theprinciples and procedures described.

FIG. 8 illustrates an alternate isolated planar resonator gyroscopeembodiment comprising four masses vibrating in plane. In thisembodiment, the resonator 800 comprises a plate including foursubresonator mass elements 802A-802D (generally referenced as 802) eachoccupying a separate pane of a supporting frame 804. The frame isattached to a baseplate (not shown) at a central support 806. Eachsubresonator mass element 802A-802D, is attached to the frame 804 by oneor more support flexures 808. In the exemplary resonator 800, foursupport flexures 808 each having a meander line shape are attached toeach mass element 802A-802D, one attached to each of the four sides ofthe element 802. Each support flexure 808 is attached to two corners ofthe mass element 802 at its ends and attached to an adjacent side of thepane of the support frame 804 at its middle. Each mass element 802A-802Dincludes eight groups of linear electrodes 810 (each electrode includingtwo elements) arranged in a pattern of increasing length from a centralpoint of the mass element 802. Each subresonator mass element 802A-802Dhas a pair of simple degenerate in-plane vibration modes to yield twodegenerate in-plane system modes involving symmetric motion of all fourelements 802A-802D, suitable for Coriolis sensing. Note that despite thesignificant differences in the architecture between this embodiment andthat of FIGS. 1A-1B, both still utilize planar mechanical resonators forsubstantially in-plane vibration with internal excitation and sensingelectrodes.

FIGS. 9A and 9B illustrate a model of the Coriolis sensing modes of thefour masses vibrating in plane. There are some key advantages of thisalternate in-plane design embodiment over other out-of-plane gyros. Forexample, this embodiment includes a central support 806 bond thatcarries no vibratory loads, virtually eliminating any possible friction.In addition, simultaneous photolithographic machining of the resonatorand electrodes can be achieved via the slots. Further, with thisembodiment diametral electrode capacitances can be summed to eliminatevibration rectification and axial vibration does not change capacitanceto a first order. The modal symmetry is largely determined byphotolithographic symmetry, not wafer thickness as with gyros employingout-of-plane vibration. Also, this embodiment employs isolation andoptimization of the sense capacitance (e.g., the outer slots of eachelement) and the drive capacitance (e.g., the inner slots of eachelement) and provides a geometrically scalable design to smaller/largerdiameters and thinner/thicker wafers. This embodiment can also beentirely defined by slots of the same width for machining uniformity andsymmetry. Finally, with this embodiment four-fold symmetry applies toall the Si crystal orientations and an ideal angular gain approachesone.

Wiring can be photographed onto the baseplate and wirebond outside thedevice to a wiring interconnect grid on a ceramic as discussed above.However, implementation of this alternate embodiment can require manyelectrodes and interconnect wiring. As discussed below, the electricalwiring for this embodiment can also be alternately developed into anintegral vacuum housing produced simultaneously with the resonator. Suchan implementation is detailed hereafter.

There is also a potential interaction with degenerate pairs of othersystem modes very close in frequency. However, these can be ignored ifnot coupled. In addition, the central support and frame compliance canbe modified to shift any coupled modes away in frequency.

This embodiment can also employ final mechanical trimming with laser orFIB of all mesogyro designs can be used to achieve full performance overthermal and vibration environments. This technique is described in U.S.application Ser. No. 10/285,886 filed Nov. 1, 2002 by Kubena et al.,which is incorporated by reference herein. It is also noted that theisolation of the degenerate modes used in present embodiments of theinvention can also be trimmed electrostatically, as with the embodimentsdiscussed above and other out-of-plane gyro designs.

This alternate embodiment can also be packaged using conventionaltechniques. However, as discussed hereafter, the resonator can also beintegrated into a novel integrated vacuum housing formed by resonatorbaseplate, case wall and readout electronics.

6.0 Isolated Planar Resonator Gyroscope with Integral Case and ReadoutElectronics

In further embodiments of the invention, the isolated planar resonatorwith internal excitation and sensing electrodes can be produced in anovel gyroscope including integral vacuum housing that is developedalong with the resonator. These further embodiments build upon the basicproduct and manufacturing procedure detailed above with respect to FIGS.6A-7.

FIG. 10A illustrates development of a vacuum cavity wall 1000 along withthe resonator 1002 and electrodes 1004. The vacuum cavity wall 1000 isproduced by first etching a continuous low wall 1006 on the baseplate1008 around the perimeter of the electrode pillars as well as a centralresonator support pillar. A bonding line 1010 (e.g., Au) is thenproduced on the low wall 1006 (over the SiO₂ layer) along with thebonding pads of the electrodes and central support. The resonator 1002is prepare with a matching bonding line that is fused to the bondingline 1010. The vacuum cavity wall 1000 is then through etched from theresonator wafer along with the resonator 1002 and the electrodes 1004.Another bonding line 1012 (e.g. Au) is produced on the top surface ofthe vacuum cavity wall 1000.

FIG. 10B illustrates development of an end cap wafer 1014 that can beused to vacuum seal a resonator 1002. Like the baseplate 1008 of FIG.10A, the end cap wafer 1014 is etched with a continuous low wall 1016. Ametal bonding line 1018 (e.g., Au) is also produced over the oxide layeron the end cap wafer 1014.

FIG. 10C illustrates the integration of the end cap wafer 1014 with thebaseplate including the vacuum cavity wall 1000. The bonding line 1010of the vacuum cavity wall 1000 and the bonding line 1018 of the end capwafer 1014 are then fused to form a vacuum seal 1020. The development ofthe vacuum cavity wall 1000 along with the planar resonator eliminatesthe need for separate hardware packaging procedures and costs. Infurther embodiments, this aspect of the present invention furtherintegrates the device and packaging solution.

FIG. 10D illustrates a further embodiment with electrical feedouts tothe excitation sensing and bias electrodes provided on the end capwafer. The end cap wafer 1014 is etched to include electrode pillars1022. Similar to the embodiment shown in FIG. 6E with metal traces 638(e.g., Au) formed on the baseplate wafer 600, metal traces 1026 areformed on the end cap wafer 1014 connected to fused bonding pads 1024 ofthe top of the electrode (formed along with the bonding line 1012) andof the electrode pillar 1022 (formed along with the bonding line 1018).In order to electrically isolate passage through the vacuum seal 1020, asecond SiO₂ layer 1016 is applied over the metal trace 1026 at least inthe area that passes through the seal 1020. Although the electricalconnection is shown to only two electrodes, other or all electrodes 1004of the resonator 1002 can be so connected, patterned onto the surface ofthe end cap wafer 1014 to pass through the finished vacuum cavity wall1000. Alternately, more space can be provided between the vacuum cavitywall 1000 and the resonator so that vertical connect pins can bearranged to pass through the end cap wafer 1014 from a point within thevacuum cavity wall 1000, similar to the arrangement shown in FIG. 6Ewith the baseplate wafer 600. Also, the electrodes 1004 which are usedconnected to metal traces 1026 on the end cap wafer 1014 can beconsidered “vertical connect pins” because they can be used to connectmetal traces on the baseplate wafer 1008 with metal traces 1026 on theend cap wafer 1014.

FIG. 10E illustrates yet another embodiment where the end cap wafer 1014includes the control electronics 1028 for the gyro. The end cap wafer1014 can be produced including the control electronics 1028, e.g. anCMOS application specific integrated circuit (ASIC) device. Applicationof the control electronics 1028 to the end cap wafer 1014 includesapplying metal traces 1032 (e.g., Al) over a first SiO₂ layer 1030. Themetal traces 1032 lead from the control electronics 1028 under thebonding line 1034 for the vacuum seal to contact pads 1036 whichcommunicate electrical power and control and rate signals to and fromthe control electronics 1028. Alternately, the metal traces 1032 can beeliminated in favor of vertical connect pins (not shown) which passthrough the end cap wafer 1014. In this case contacts are provided onthe top side of the integrated device. A second SiO₂ layer 1038 isapplied over the metal traces 1032 of the control electronics 1028 toisolate a second layer of metal traces 1040 which couple the controlelectronics 1028 to the excitation, sensing and bias electrodes throughbonding pads 1042.

FIG. 10F illustrates the integrated end cap wafer 1014 including thecontrol electronics 1028. As with previous embodiments, the bonding line1034 is fused to form the vacuum seal 1020 around the perimeter of theintegrated device. Similarly, the bonding pads 1042 are fused to thevarious excitation, sensing and bias electrodes 1004.

The specific feature of the device design that enables relativelyseamless subsequent integration with the CMOS control electronics is thepresence of the “vertical connect pins” that bring signal lines from thebaseplate wafer where the connections to the electrodes are made to thetop surface of the device wafer pair. The pins can be connected to thefeed-through lines on the end cap wafer or to the relevant metallizationon the CMOS electronics during the bonding process. The bonding processwhich makes these electrical connections also creates the vacuum sealedcavity that houses the gyroscope resonator. The bonding can be performedwith a solder reflow (such as Au—Sn) which ensures a vacuum-tight sealand a high quality electrical connection.

A proper choice of the device wafer pair and end cap wafer/device pairbonding methods is important to both ensure a tight vacuum seal and tomaintain electrical connectivity and mechanical integrity. Inparticular, the device wafer pair should be bonded with a highertemperature process, such as a Au thermal compression or Au—Si eutectic,while the readout electronics wafer should be bonded with a lowertemperature process, such as Au—Sn or Au—In. This is done to maintainthe mechanical integrity of free-standing electrodes during the readoutelectronics wafer bonding phase.

Since the planar resonator to baseplate bond can be accomplished by arobust Au/Si eutectic bond, a solder seal ring approach can be used forend cap wafer to resonator wafer bond with a melting point ofapproximately 300° C. This will allow the gyro to operate in atemperature environment as high approximately 200° C. if needed. Thesilicon planar resonator wafer and baseplate pair can be bonded directlyto a Si readout electronics wafer containing CMOS control electronics inorder to reduce the induced capacitive variations in a highg-environment. By connecting the gyro sense and control electrodesdirectly to the control electronics on the Si readout electronics waferusing Au/Si eutectic bonding, the overall robustness to high g-loadingand thermal variations can be increased. In addition, since the gyrostructure will form part of the readout electronics wafer, theelectronics integration and the wafer vacuum encapsulation isaccomplished in one fabrication step.

The foregoing description of the preferred embodiment of the inventionhas been presented for the purposes of illustration and description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed. Many modifications and variations are possiblein light of the above teaching. It is intended that the scope of theinvention be limited not by this detailed description, but rather by theclaims appended hereto. The above specification, examples and dataprovide a complete description of the manufacture and use of theinvention. Since many embodiments of the invention can be made withoutdeparting from the scope of the invention, the invention resides in theclaims hereinafter appended.

1. A method producing an inertial sensor, comprising the steps of:providing a planar mechanical resonator for substantially in-plane soliddisc vibration with two isolated and degenerate resonator modes forCoriolis sensing having a central mounting point; supporting the planarmechanical resonator at the central mounting point; providing at leastone excitation electrode within an interior of the resonator to excitevibration of the isolated and degenerate resonator modes; and providingat least one sensing electrode within the interior of the planarmechanical resonator for sensing the vibration of the isolated anddegenerate resonator modes.
 2. The method of claim 1, wherein planarresonator vibration is substantially isolated from the central mountingpoint.
 3. The method of claim 1, wherein the in-plane solid discvibration comprises substantially radial motion about the centralmounting point.
 4. The method of claim 1, further comprising a baseplatesupporting the central mounting point, excitation electrodes and sensingelectrode.
 5. The method of claim 4, wherein the sensor is produced byetching the baseplate, bonding a wafer to the etched baseplate andthrough-etching the wafer to form the planar mechanical resonator,excitation electrode and sensing electrode.
 6. The method of claim 5,further comprising grinding and polishing the through-etched wafer to adesired thickness.
 7. The method of claim 5, wherein through etching thewafer also forms a case vacuum wall for the inertial sensor.
 8. Themethod of claim 1, wherein the planar mechanical resonator comprises aplurality of slots.
 9. The method of claim 8, wherein the plurality ofslots are arranged in an annular pattern around the central mountingpoint.
 10. The method of claim 8, wherein the plurality of slots arearranged with substantially uniform radial spacing around the centralmounting point.
 11. The method of claim 8, wherein the plurality ofslots are arranged in a symmetric pattern.
 12. The method of claim 8,wherein the at least one excitation electrode is disposed within one ormore of the plurality of slots.
 13. The method of claim 12, wherein theat least one excitation electrode is disposed within one or more innerslots of the plurality of slots.
 14. The method of claim 8, wherein theat least one sensing electrode is disposed with one or more of theplurality of slots.
 15. The method of claim 14, wherein the at least onesensing electrode is disposed within one or more outer slots of theplurality of slots.
 16. The method of claim 1, wherein the planarmechanical resonator comprises four masses, each having a simpledegenerate pair of in-plane vibration modes.
 17. The method of claim 16,wherein the planar mechanical resonator has two degenerate in-planesystem modes producing symmetric motion of the four masses for Coriolissensing.
 18. The method of claim 1, further comprising providing anintegral case vacuum wall formed from a same wafer as the planarmechanical resonator.
 19. The method of claim 1, further comprisingproviding an end cap wafer bonded to a case wall with a vacuum seal. 20.The method of claim 19, wherein the end cap wafer includes readoutelectronics for the inertial sensor.